Lips are a visible organ, at the mouth of humans and many animals. Lips are soft, protruding, movable, and serve as the opening for food intake, as an erogenous organ used in kissing and other acts of intimacy, as a tactile sensory organ, and in the articulation of speech.
Anatomical basics of the human lip
One differentiates between the Upper (Labium superius) and lower lip (Labium inferius). The lower lip is usually somewhat larger. The border between the lips and the surrounding skin is referred to as the vermilion border, or simply the vermilion. The vertical groove on the upper lip, is known as the philtrum. The entire skin between the upper lip and the nose is referred to as the "ergotrid".
The skin of the lip, with three to five cellular layers, is very thin compared to typical face skin, which has up to 16 layers. With light skin color, the lip skin contains less melanocytes (cells which produce melanin pigment , which give skin its color). Because of this, the blood vessels appear through the skin of the lips, which leads to their notable red coloring. With darker skin color this effect is less prominent, as in this case the skin of the lips contains more melanin and thus is visually thicker.
The lip skin is not hairy, and does not have sweat glands or sebaceous glands. Therefore it does not have the usual protection layer of sweat and body oils which keep the skin smooth, kill pathogens, and regulate warmth. For these reasons, the lips dry out faster and become chapped more easily.
Anatomy in detail
The skin of the lips is stratified squamous epithelium. The mucous membrane is represented by a large area in the sensory cortex, and is therefore highly sensitive. The Frenulum Labii Inferioris is the frenulum of the lower lip. The Frenulum Labii Superioris is the frenulum of the upper lip.
Sensory nerve supply
The infraorbital nerve is a branch of the maxillary branch. It supplies not only the upper lip, but much of the skin of the face between the upper lip and the lower eyelid, except for the bridge of the nose.
The mental nerve is a branch of the mandibular branch ( via the inferior alveolar nerve). It supplies the skin and mucous membrane of the lower lip and labial gingiva (gum) anteriorly.
The facial artery is one of the six non-terminal branches of the external carotid artery. It supplies the lips by its superior and inferior labial branches, each of which bifurcate and anastomose with their companion artery from the other side.
Muscles acting on the lips
The muscles acting on the lips are considered part of the muscles of facial expression. All muscles of facial expression are derived from the mesoderm of the second pharyngeal arch, and are therefore supplied (motor supply) by the nerve of the second pharyngeal arch, the facial nerve (7th cranial nerve). The muscles of facial expression are all specialised members, of the panniculus carnosus, which attach to the dermis and so wrinkle, or dimple the overlying skin. Functionally, the muscles of facial expression are arranged in groups around the orbits, nose and mouth.
The muscles acting on the lips:
sphincters of the oral orifice
anchor point for several muscles
levator labii superioris
levator labii superioris alaeque nasi
levator anguli oris
depressor anguli oris
depressor labii inferioris
Functions of the lips
Because they have their own muscles and bordering muscles, the lips are very movable. Lips are used for eating functions, like holding food or to get it in the mouth. In addition, lips serve to close the mouth airtight shut, and to, hold food and drink inside, and to keep out unwanted objects. Through making a narrow funnel with the lips, the suction of the mouth is increased. This suction is essential for babies to breast feed. Lips can also be used to suck in other contexts, such as tactile stimulation of other people.
Because of their high amounts of nerve endings, the lips are an erogenous zone. The lips therefore play a crucial role in kissing and other acts of intimacy. As the mouth, lips, and tongue are among a woman's most erogenous zones, stimulating a woman's mouth and lips during acts of intimacy has been shown to elicit pleasure and to have a direct stimulatory effect on arousing her genital organs. 
A woman's lips are also a visible expression of her fertility. In studies performed on the science of human attraction, psychologists have concluded that a woman's facial and sexual attractiveness is closely linked to the makeup of her hormones during puberty and development. Contrary to the effects of testosterone on a man's facial structure, the effects of a woman's estrogen levels serve to maintain a relatively "childlike" and youthful facial structure during puberty and during final maturation. It has been shown that the more estrogen a woman has, the larger her eyes and the fuller her lips. Surveys performed by sexual psychologists have also found that universally, men find a woman's full lips to be more sexually attractive than lips that are less so. A woman's lips are therefore sexually attractive to males because they serve as a biological indicator of a woman's health and fertility. As such, a woman's lipstick (or collagen lip enhancement) takes advantage by "tricking" men into thinking that a women has more estrogen than she actually has, and thus that she is more fertile and attractive.
The lip has many nerve endings and reacts as part of the tactile (touch) senses. Lips are very sensitive to touch, warmth, and, cold. It is therefore an important aide for exploring unknown objects for babies and toddlers.
The lips serve for creating different sounds - mainly the labial, bilabial, and labiodental consonant sounds - and thus create an important part of the speech apparatus. The lips enable whistling and the performing of wind instruments such as the trumpet, clarinet, and flute.
See Full Article: facial expression. The lips visibly express emotions.
Lips are often viewed as a symbol for sensuality and sexuality. This has many origins; above all, the lips are a very sensitive erogenous and tactile organ. Reproductive psychologists have suggested that one reason the female lips are seen as sexually attractive might be because they mimic the appearance and sexual swelling of the labia of the vulva, and that a woman's lips are effectively a secondary sexual organ. This is consistent with research showing male sexual attitudes towards a woman's lips and mouth to be very similar to those towards a woman's vulva. Furthermore, in many cultures of the world, a woman's mouth and lips are veiled because of their representative association with the vulva, and because of their role as a woman's secondary sexual organ.
As part of the mouth, the lips are also associated with the symbolism associated with the mouth as orifice by which food is taken in. The lips are also linked symbolically to neonatal psychology(see for example oral stage of the psychology according to Sigmund Freud).
Changes to the lip
One of the most frequent changes of the lips is a blue coloring due to cyanosis; the blood contains less oxygen, and thus has a dark red to blue color, which shows through the thin skin. Cyanosis is the reason why corpses always have blue lips. In cold weather cyanosis can appear, so especially in the winter, blue lips may not be an uncommon sight.
Lips can (temporarily) swell. The reasons for this are varied and can be from sexual stimulation, injuries and side effects of medications, or misalignment of teeth.
Cracks or splits in the angles of the lips could be the result of an inflammation of the lips, Angular cheilitis.
As an organ of the body, the lip can be a focus of disease or show symptoms of a disease:
Lip herpes (technically Herpes labialis, a form of herpes simplex) is a viral infection which appears in the formation of painful blisters at the lip. It's also commonly known as a cold sore.
Carcinoma (a malignant cancer that arises from epithelial cells) at the lips, is caused predominantly by using tobacco and overexposure of sunlight. To a lesser extent, it could also come from lack of oral hygiene or poor fitting dentures. Alcohol appears to increase the carcinoma risk associated with tobacco use.
Tomiyama N, Ichida T, Yamaguchi K (2004). "Electromyographic activity of lower lip muscles when chewing with the lips in contact and apart". Angle Orthod 74 (1): 31–6. PMID 15038488.
Bisson M, Grobbelaar A (2004). "The esthetic properties of lips: a comparison of models and nonmodels". Angle Orthod 74 (2): 162–6. PMID 15132441.
McMinn, R. M. H.; Last, R. J. (1994). Last's anatomy, regional and applied. Edinburgh: Churchill Livingstone. ISBN 0-443-04662-X.
^ Bechtel, Stefan, and Larry Stains. Sex, A Man's Guide, 95.
^ Attractive women tend to have higher oestrogen levels. Retrieved on 2007-12-12.
^ Why do men find big lips and little noses so sexy? I'll paint you a picture - Comment - Times Online. Retrieved on 2007-12-12.